There has been a seat device that includes a seat movable along a rail fixed to a floor of a vehicle such as an automobile, and a lock mechanism disposed to fasten the seat at any position on the rail. In this type of technique, there has been a seat device where an element that generates a signal is installed on the rail while a detector for detecting the signal is installed on the seat, so as to detect the position of the seat on the rail (Patent Literature 1).